In modern day communication systems, digital data is employed for transmission via a switching matrix to various subscribers associated with the system. The use of digital data enables efficient transmission and enables the various system modules to be implemented by integrated circuit techniques which are particularly well suited for the fabrication of digital circuitry.
In a digital telephone system, one converts analog or voice data to digital signals for processing and transmission via a switching matrix. The digital signals are converted back to analog signals to enable subscribers of the system to conventionally communicate. The digital telephony system enables one to implement subscriber line circuits mainly by employing digital circuitry which, as indicated, can be integrated as by LSI, large scale integrated circuits and hence, offers substantial reduction in costs and efficient and reliable operation.
The conversion of signals to analog or digital signals necessitates the use of both digital to analog converters and analog to digital converters. In any event, due to the cost considerations in any telephone system, one must be concerned with economics as well as reliable operation consistent with modern day telephony requirements.
It has been determined that an increase in the sampling or word rate associated with a reduction in the number of bits will enable one to realize simpler and inexpensive configurations for digital to analog converters and hence, reduce hardware costs. This is a prime concern in a telephone line circuit as one such circuit is required for each subscriber.
The increase in word rate as described above has generally been referred to as interpolation with the apparatus for achieving the results referred to as an interpolator. If reference is made to U.S. Pat. No. 4,109,110 entitled DIGITAL TO ANALOG CONVERTER issued on Aug. 22, 1978 to M. J. Gingell, examples of various structures are shown for performing interpolation in conjunction with digital to analog conversion. The advantages of the interpolation technique are also described.
It is apparent that interpolation as well as conversion should be accomplished as economically as possible, while providing reliable operation. In the prior art, one form of an interpolator is referred to as a "triangular window" or a linear interpolator, which devices have been used in conjunction with various types of digital to analog converters. These devices require a great deal of digital circuitry and hence, are associated with relatively large processing delays. Such delays introduce distortion and interference in the processed signals and are undesirable.
An interpolator should also operate to attenuate frequencies outside of the desired analog band to assure that the digital signals are free of interfering frequency components and hence, to assure that the retrieved analog signal is an accurate replica of the original signal.
It is, of course, desirable that the interpolator be capable of accommodating a plurality of different word sizes for different digital coding schemes, with a low processing delay. In prior art interpolators, multipliers were required as rate multipliers and so on. These devices are relatively complicated digital devices and require a great deal of hardware for implementation. Hence, an interpolator according to this invention is provided which eliminates the need for a multiplier.
The interpolator according to this invention provides extremely high attenuation at interfering frequencies, while capable of accommodating flexible word sizes with low processing time delay. As will be described, the interpolator uses relatively economical digital circuitry which is less complex than that required for prior art interpolators. These and other features such as low passband deviation are afforded by employing the interpolator according to this invention.